Electrolytic cell



27, 1951 c. CARTER ET AL 2,542,989

ELECTROLYTIC CELL.

Filed Sept. 9, 1946 3 Sheets-Sheet l 2\ John V. 8. Glass d FIG. 2 Charles Carter IN VEN TORS M mm 1951 CARTER ET AL 2,542,989

ELECTROLYTIC CELL Filed Sept. 9, 1946 3 Sheets-Sheet 2 7 I 4 I i III/III John V. 8. Glass d Charles Carter IN VEN T ORS Feb. 27, 1951 c. CARTER ET AL 2,542,989

ELECTROLYTIC CELL Filed Sept. 9, 1946 I :5 Sheets-Sheet s g 4 FIG.6

Patented Feb. 27, i951 UNITED STATES PATENT OFFICE ELECTROLYTIC CELL Charles Carter, Liverpool, and John V. S. Glass,

Frodsham,

England, assignors to Imperial Chemical Industries Limited, a corporation of Great Britain Application September 9, 1946, Serial No. 695,622 In Great Britain-August 27, 1941 Section 1, Public Law 690, August 8, 1946 Patent expires August 27, 1961 trolyte having as one electrode a heavy liquid such as a sheet of mercury which flows over a horizontal or slightly inclined surface, with a second electrode formed of plates or blocks of solid material, e. g., carbon or graphite, arranged a short distance above the liquid electrode with the under-surface substantially parallel to the surface of the liquid electrode. Such cells are referred to as electrolytic cells of the liquid electrode type.

An example of such a cell is one for the electrolysis of brine in which the cathode is a continuous sheet of mercury flowing over a slightly inclined plane surface forming the bottom of a trough-shaped vessel, and the anode is formed of a number of plates of graphite immersed in the brine and suspended by graphite rods from one or more lids forming the cover of the cell. As a result of the electrolysis the mercury becomes an amalgam continuously increasing in strength as electrolysis proceeds, so that only at the beginning of its path is the electrode the chemical element mercury. Therefore, the word mercury as used in this specification and its appended claims will be understood to cover the resulting amalgams as well as the pure metal unless the context requires the narrower meaning. In operation the under-surfaces of the graphite anode plates wear away and the electrical resistance of the cell thus increases so that it is essential to have means for adjusting the height of the electrodes to compensate for this wear.

A principal object of this invention is the provision of electrolytic cells of the liquid electrode type of new form and design. A further object is the provision of electrolytic cells of the subject type provided with means external of the cell for adjusting the solid electrode position while the cell is in operation. Another object is the provision of liquid-electrode, electrolytic cells which may be operated at superatmospheric pressures. A still further object is the provision of such cells which operate at relatively low voltage and which make possible very close control upon the reactions taking place within the cell and the efficiency of the process. An additional object is the provision of such cells in which the inter-electrode gap 9 Claims. (01. 204-225) may be changed or varied without fluid escaping from the cell even when the cell is operated under super-atmospheric pressure. Another ob-.

ject is the provision of such cells which permit the electrolyte flow between the inter-electrode; gap to be readily controlled both as to quantity and velocity. Other objects and the entire scope of applicability of this invention will become apparent from the detailed description given hereinafter.

These objects are accomplished according to the present invention by providing in an elec-- trolytic cell of the liquid electrode type com Advantageously, the external adjusting meanscomprises a plurality of screw threaded supports adapted on rotation to cause a vertical movementof the upper part of the cell relative to the lower part.

According to a further feature of the inven-' tion a particularly advantageous sealing means comprises a resilient deformable packing located beneath mercury.

The apparatus of this invention and its details of construction may be more readily com-' prehended by reference to the accompanying more or less diagrammatic drawings, not to scale,"

in which:

Figure 1 is a vertical section of one form of the electrolytic cells of this invention.

Figure 2 is a vertical section of another form of our electrolytic cells in which the sealing means of the cell consists of a flexible seal attached to the side of the lid and the base of the cell.

Figure 3 is a vertical section of another specific embodiment of our cells.

Figure 4 is a vertical section of a cell in which the cover of the cell may be moved positively.

by applied force in both vertical directions.

directions and with the rod supporting the anode.

supplied with means to prevent exudation of electrolyte from the end of the rod. Such means of preventing exudation of electrolyte are described and claimed in copend ing application Serial No. 695,624, filed September 9, 1946 by Arthur Wesley Ravenscroft, entitled Electrode supports for electrolytic cells.

Figure 6 is a vertical section of another form with appropriate means (not shown) for supplying mercury and fresh electrolyte to the cell, for removing amalgam and spent electrolyte, for removing chlorine evolved in. the electrolysis, and forsupplying electric current to the electrodes. These appropriate inlets and outlets may take the customary form, and in the case of the, forms of the invention shown in. Figures i-and 5 these inlets and outlets for the liquids will pass through the bottom of the cell to terminate as orifices within the area bounded by the peripheral channels.

Each cell has a lid or cover 4 carrying a number of solid carbon anode blocks 5 of known type (one of which is shown), each depending from a carbon rod 8 which passes through an opening in the lid, and through which current is supplied to the anode by means shown only in Figure 5. In the drawings the joint between the lid 4 and the rod 6 is sealed :by a sealing composition 1, but any other suitable method of rendering the joint gas-tight may be employed. From each side of the lid 4 project a number of flanges 8 (one on each side being visible in the drawings), in which flanges in Figuresl to 3 are threaded holes engaging with threaded bolts 9. In Figures 1 to 3 the bolts are shown to rest on lugs II] projectin from the sides of the cell. This trough-shaped vessel I--may be made of concrete or of steel, the latter case being appropriately coated with ebonite or other material which will protect the steel against the corrosive action of the electrolyte and of the chlorine. The lid- .may also be made of concrete, slate, covered steel or the like.

Referring now more particularly to Figure 1, the sides of the cell body have around their upper edges a trough-shaped portion I I in which is a layer of plastic, luting composition I2. When the cell is to be put into operation the anode blocks are fixed to the lid in the correct position to give more than the appropriate gap between the lower faces of anode blocks and the bottom of cell I, and. thus between those lower faces and the surface of the mercury -2 when the cell is in operation, and the lid is then put in position so that the bolts 9 rest on lugs I0. The exact height of the under-surface of the anode above the bottom of the trough is then determined, as by "calculation from the predetermined dimensions of. the trough of the anode-lid assembly and of the vertical distance between members 8 and ID. The filter-electrode gapv is then adjusted accordingly by rotating bolts 9 so as to raise the heads of the bolts by a distance equal to that by which the anode has ing lug Ill. The cell is then ready for use.

large. From a measurement of the increases in potential drop across the inter-electrode gap and the known specific resistance of the electrolyte, it can be calculated by what amount it is necessary to lower the anode-lid assembly, and this is then effected while the cell is in operation by rotating bolts 9 so as to raise the heads of the bolts by that amount, and the lid is pressed down so that the bolts again touch the corresponding supporting member III. The original efiiciency of the cell will then be restored, butin time a further adjustment will be needed, and this can then be efiected in similar fashion.

Referring. now to Figure 2, in place of the member II and plastic composition I2 of Fig. 1 there is provided all around the cell where the gap I3 occurs between lid 4 and trough I an endless band I4 of a flexible material, having a resistance to chlorine, e. g., polyvinyl chloride, polythene, or chlorinated polythene, which is sealed to the walls of vessel I. When the lid 4 has been lowered into its approximate position, as described in connection with Figure 1, this strip is also sealed in the same way to the periphery The fine adjustments to the position of lid 4. of the anode are then made as described in connection with Figure 1, except that the lid will sinkunder its own weight as the bolts 9 are raised, and in so doing will bend the flex ible material I4. Adjustments during the running of the cell can also be made in the same manner.

Referring to Figure 3, the external adjustingmeans for adjusting the height of the anode is similar to that of the cell structure forms shown in Figures 1 and 2 consisting of bolts 9 which extend through the threaded flange 8 and rest upon the lugs extending from the cell body I. The sealing means in this embodiment of the invention consists of a rigid skirt IT, integral with the lid 4 which extends downwardlyinto a channel [5 in the cell-body walls which is adapted to receive the skirt H. In the base of the channel I5 positioned a deformable resilient packing I5 such as an inflated rubber tube which is covered on both sides of the skirt by suitable material Ila, e. .g., mercury, or an oil resistant to attack by chlorine.

In the form of the invention depicted in Figure 4l,the vessel I is formed with a narrow deep channeled I5 extending the whole way around the plane surface on which mercury 2 flows. In the bottom of the channel I5 is an inflated rubber tube It and the lid 4! has .a rigid skirt I? integral with the lid and extending downwards far enough to dip into the channel I5 and to rest on the tube I6 when anodes 5 are in position. In putting such a cells into operation the bottom of the vessel I is first flooded with mercury so as to fill channel 15,, and thus cover the tube I6; the lid 4 is then caused to go into place by means of the bolts 9a in a position which gives approximately the correct inter-electrode gap. The bolts 9a are threaded into inserts 9b which are contained in the top of the "sides of the housing I. The skirt '1 will then rest on tube It and thus seal the space inside the cell from outside. Adjustments in the height of the anode can be made as'required'by movement of the bolts 9a; if bolts 9a are lowered the positive downward thrust on the anode assembly will cause the skirt ll to deform rubber tube i6 and thus allow the inter-electrode gap to decrease. II", initially, the inter-electrode gap is too small it can be increased by raising bolts ea in the lugs 8, and the resilience of tube it ensures that the seal will still be maintained. Adjustments can be made in a similar manner during the use of the cell when the anode gap becomes too large as the result of wear. l

In the embodiment shown in Figure 5, the cell is provided with a lid which is supported by a plurality of-hydraulic rams disposed at intervals around the perimeter of the lid, the vertical adjustment of the lid being achieved by supplying or removing hydraulic pressure from the rams Depending from lid 4 is a skirt ll, the lower edge of which dips into the channel l5, and an endless inflated rubber tube llia is disposed between the inner wall of the channel and the skirt When the cell is in operation the channel will be filled with mercury, and the assembly of the skirt i? and tube ltd will form a seal reinforced by the mercury; the rubber tube will thus be protected from the action of the electrolyte, which will contain dissolved chlorine, by the layer of mercury above it.

From each side or" lid l project laterally a number of flanges 8 (one on each side being shown), each of which is attached to the upper end of a'piston rod it of a hydraulic ram it]. Each ram is fitted with a pipe above the upper limit of travel of the piston 2| and pipe 22 below the lower limit of travel of the piston. By means of appropriate two-way valves and an appropriate piping system, water under pressure can be supplied to the ram through each of the pipes 2R3 while water is allowed to leak away from below the pistons 2i throu h pipes 22, thus lowering the lid 4, and thus anode 5. On the other hand water can be supplied to the ram through all the pipes 22 and allowed to escape from pipes 29 thus raising thelid =l and anodes 5.

Figures 6 and 7 illustrate modified forms of means external from the cell for adjusting the height of the anode of the cell relative to the liquid electrode. 'In Figure 6 there is shown a portion of the side of the cell body i provided with alug 'A wedge-shaped member 2A rides upon the lug 23 and is caused to move horizontally toward and away from the side of the cell bymeans of the bolt 25. A flange or lug 26 provided with a downwardly bent section which 9 outer periphery of the cover i and the top edge 9;

The base i in this case is madeof the base I. from iron and is provided with a lining 2'! which may be made from any chlorine resistant mate-- rial such aspolyvinyl chloride, polythene, or

chlorinated polythene.

In the embodiment shown in Figure E is provided with a bracket 23 on which is journalled a cam 28.

29. The upper edge of the base I is provided with a trough 35) in which rests a portion of sealing composition 3! for sealing the gap between the cover 4 and theupper edge of the base I. Adjustment of the height of the anode relative to the liquid cathode is, obtained by rotating the.

7, the base The flange 3 extending from the cover 4 r sts upon the outer edge of the cam} 6; cam 29, the weight of the cover 4' and there mainder of the apparatus associated therewith being sufiicient to cause the cover 4 to compress the sealing COI'IIDOSlUlOIliH and cause the cell to 1 be sealed from the surrounding atmosphere.

Referring again in detail to rigure b, there canbeseen novel means for preventing exudation of electrolyte from the anode supporting rod which may be occasioned because 01' .tne cell being opierated at super-atmospheric pressures.

the upper end of the rod is threaded to receive a ring :12 of metal, ebonite, or other suitable ma-' terial, which carries a stirrup disposed immediately above the end of the rod, and a circular g 'metal cap 2-34 rests with its rim on the upper surface of the ring screwed on to the rod; ii desired a packing ring 35 of cork, rubber, or other come pressible material may.- be interposed between the. cap and the metal rim. The stirrup carries a set gg screw 36 adapted to bear against the top of the cap and to be rendered immovable by locknut 3h The space between the cap and the top or the anode is filled with a grease-like composition;

this may be, for example, a grease-like mixture of a chlorinated naphthalene and a chlorinated paramn, wax, or a viscous chlorinated paraffinwax thickened by an addition of chlorinated rubher; a hydrocarbon grease could also be used.

In use, the set screw in the stirrup is made to 30 press against the top of the cap, thus pressing the grease-like composition into the pores in the surface layers of the rod; as a result, electrolyte seeping up the pores oi the rod will meet a barrier oi grease-like material, and exudation of the elec- Operation of the cells which use a sealing com position interposed in a trough-like unit on the top edge of the sides of the base, e. g., forms of the screw-adjusting means; the gap between the U0 ing in a plastic luting composition. If the interelectrode gap is found to be toolarge the screwadjusting means can be actuated to allow the gap to be reduced by the appropriate amount. If the screw-adjusting means provides positive move-1 ment of the lid towards the vessel the plastic, composition will thus automatically bedeiormed j. and the solid electrodes brought to the correct position. However, if the screw-adjusting means I gives positive movement of the lid only in the one I direction, (upwards), the necessary adjustment. can be made by screwing thebolts in the appropriate direction by the amount by which it is desired to lower the electrodes, whereupon,-if thesealing composition is sufficiently plastic, the lid.

will sink by its own weight until the-bolts are again resting on the ledge. too stiff for this purpose, the lid can then be pressed down by hand pressure or otherwise until, the result is achieved. The amount by which the, lid is lowered may be chosen in accordance with the known rate of wear of the electrodes (under given operating conditions) or by other methods; 1; for example, it may be chosen so that the cel 5 voltage is kept between specified limits. t

apparatus such as shown in Figures 1 and '7, is" commenced by setting the lid slightly above the highest position it will need to occupy by moving lid and the top of the cell is then closed by pack If the composition is when the electrodes require renewal the lid is removed, the electrodesfixed inposition, the lid replaced and the sealing composition renewed, preferably after making at least the preliminary adjustments intheheight of the lidby means of the bolts.

In the forms of the invention using the-integral skirt. on. the cell lid combined with a seal. of resilient packing and mercury or other sealing liquid such as oil,,the rubber tube should not be inflated so hard that it cannotbe deformed, but neverthelesssulneient for it to. be resilient. (in supplying mercury to the cell the space in the channel. above the rubber tube isfilled. with mercury. and the rubber isthusprevented from coming into contact with gaseou chlorine, or with brine. containing dissolved chlorine, thereby deteriorating and. losing its resilience. By providing. the resilient deformable packing in. the channel the diversion of. the mercury flow from the plane. surface to the channel can. be reduced to smallproportions. It. will be apparent thatthis latter desirable condition can more readily be realized if the thickness of the sides of the cover sonly slightly less than the width of the chan neL In. making. an. adjustment to the position of the lid, it is merely necessary to reset the position of the nuts onwhichv the lugsof the lid rest, and the. weight of thelatter will cause the rubber packing in. the channel. to. be. deformed as the lid. is lowered. If desired, the adjustment may thusbe madewithoutbreaking the sealand. may evenbe; made without interrupting the electrolysis. To guardagainst the generation of sumcient pressure inside the cell to cause the lid to press pwards. instead of downwards, threaded rods may besecured tov the sides of the cell trough, whichrods pass through holes in the lugs of the. lid. two. nuts being then provided on each rod, one above the. lug and one. below it. The seal will then not bebroken even. if the lid is pressed upwards throushthe. pressure of chlorine in the cell.

In any of. the. above forms of the. invention, various luting compositions may be used which are relatively inert to chlorine, for example,v a plastic mixture of chlorinated parafiin wax, chlorinated polythene, neoprene and the like.

v However, the invention is not limited to the use of plastic compositions, but also includes the use of liquid seals, e g, brine, or of mercury covered with. a layer of a hydrocarbon oil to prevent attack. within. the cell by chlorine. As indicated, we may also make the seal by means of a strip of rubber or other flexible impermeable material extending over the short gap between the lid and the. wall and sealed to each by tar, pitch or other luting composition. In those forms of the invention in which the cell lid is provided with a skirt which suspends into a channel in the cell body, it has been found that usually fine operational results are obtained" where: the channel is filled with a thermoplastic substance inert to the gases evolved in the electrolysis and capable of wetting the surfaces of the skirt and channel in lieu of the mercury and. 5

deformable tubing seal described above. Electrolytic cells of the liquid electrode type in corporating a thermoplastic seal of this kind are described and claimed in copending application, Serial No. 695,625, filed Sept. 9, 1946 by Arthur Wesley Ravenscrof-t entitled Electrolytic Cells now- Patent Number 2,502,888. Suitable thermo plastic materials which may be used include normallysolid chlorinated naphthalene containing,

for example, 45.. to 55% chlorine, solid chlorinated paraffin wax, mixture of chlorinated. nabbethalene and chlorinated wax, and the like Asmany. and varied modifications of the sub--.

L. An electrolytic cell of. the liquid electrode y e having a base carrying a flowin liquid sled-.- trode, a cover on the base extending across the cell beyond the edges of: the liquid electrode, a

solid electrode rigidly supported in the cell by.

said cover, said solid electrode having a plane under-surface disposed a shortdistance above he upper suriace oi, the liquid l tro e and substantiall parallel thereto, said plane surface. extendingcontinuouslyacross the major portion oi. the cell, means, combined with the cover for adjusting the height of the cover relative to the base comprising a pluralit of horizontal ex-.

tcnsicnson saidcovcr which bear u on vertically adiustablcfi lpport members carried by said base, and means for sealing the cover to the base for llowing appr iable relative vertical movement etween the cover and the base comprising an elonga ed, re ilient element of a length. corresponding to the perimeter of said base in sealenga ement with the peripheral edges of said cover and said base.

2. In n electrolytic cellof the liquid electrode type having a base carrying a flowing liquid electrqdoi a cover on the base extending across the cell beyond theedges of the liquid electrode and a Solid. electrode having a plane under-surface rigidly supported in the cell by said cover with its undersllriace disposed at short. distance above the upper surface or the liquid electrode and substantially parallel, thereto, said solid electrode extending continuously across the major portion or the cell, the combination which comprises means combined, with the cover for adjusting the height of the cover relative to the base comprising a plurality of horizontal extensions on said cover which bear upon, vertically adjustable support members carried by said base, upwardly ex-. tending walls integral with the base, and means for sealing the cell comprising a layer of lastic composition filling a gap between the lower side of the cover. and the top edgesof the walls of the ase,

3- In anelectrolytic cell of the liquid electrode type having a base carrying a flowing liquid elec- U trode, a cover on the base extending across the cell beyond the edges of the liquid electrode and a. solid electrode. having a. plane under-surface.

rigidly supported in the cell by said cover with its under-surface disposed a short distance above the. upper surface or the liquid electrode and substantially parallelthereto, said solid electrode extending continuously across the major portion of the cell, the combination which comprises means comblnedwith the cover for a j n the height of the cover relative to the base comprismg a pluralit of horizontal extensions on said cover which. bear upon vertically adjustablesup- Port members carried by said base, and means. for sealing the cell. which comprises an endless band of chlorine impervious flexible, material surrounding a gap between the cover and the base, the oppositeedges of said band. being sealed. to the cover and base, respectively.

- 4. In an. electrolytic cell of the liquid electrode type having a base carrying a flowing liquid electrode, a cover on the base extending across the cell beyond the edges of the liquid electrode and a solid electrode having a plane under-surface rigidly supported in the cell by said cover with its under-surface disposed a short distance above the upper surface of the liquid electrode and substantially parallel thereto, said solid electrode extending continuously across the major portion of the cell, the combination which comprises meanscombined with the cover for adjusting the height of the cover relative to the base comprising a plurality of horizontal extensions on said cover which bear upon vertically adjustable support members carried by said base, a peripheral channel in said base, a skirt integral with said cover depending into said channel, a deformable, resilient packing within said channel contacting a wall of the channel and of the skirt, said packing being covered by a layer of sealing liquid contained in the channel.

5. In an electrolytic cell of the liquid electrode type having a base carrying a flowing liquid electrode, a cover on the base extending across the cell beyond the edges of the liquid electrode and a solid electrode having a plane under-surface rigidly supported in the cell by said cover with its under-surface disposed a short distance above the upper surface of the liquid electrode and substantially parallel thereto, said solid electrode extending continuously across the major portion of the cell, the combination which comprises means combined with the cover for adjusting the height of the cover relative to the base comprising a plurality of threaded bolts operatively engaging the cover and bearing against a portion of the base, and means for sealing the cover to the base in a manner allowing relative vertical movement be tween the cover and the base comprising an elongated, resilient element of a length corresponding to the perimeter of said base in sealing engagement with the peripheral edges of said cover and said base.

6. In an electrolytic cell of the liquid electrode type having a base carrying a flowing liquid electrode, a cover on the base extending across the cell beyond the edges of the liquid electrode and a solid electrode having a plane under-surface rigidly supported in the cell by said cover with its under-surface disposed a short distance above the upper surface of the liquid electrode,

and substantially parallel thereto, said solid electrode extending continuously across the major portion of the cell, the combination which comprises means combined with the cover for adjusting the height of the cover relative to the base comprising a plurality of threaded bolts operatively engaging the cover and the base, and means for sealing the cover to the base in a manner allowing relative movement between the base and cover comprising a peripheral channel in the base, a skirt integral with the cover depending into the channel, a deformable resilient packing within said channel contacting a face of the skirt and the channel wall, said packing being covered by a layer of sealing liquid contained in the channel.

7. An electrolytic cell of the liquid electrode type, which may be operated at an appreciable internal pressure and in which the inter-electrode gap may be adjusted without interruption of the cell operation, having a base carrying a flowing mercury electrode, a cover which extends substantially over the cell, a solid electrode rigidly supported in the cell by said cover, said solid electrode having a plane under-surface disposed a short distance above the upper surface of the liquid electrode and substantially parallel thereto, said plane surface extending continuously across the major portion of the cell, means combined with the cover for adjusting the height of the cover relative to the base comprising a plurality of horizontal extensions on said cover which bear upon vertically adjustable support members carried by said base, and means for sealing the cover to the base for allowing suiiicient relative vertical movement between the cover and the base to substantially alter the width of the inter-electrode space by such relative movement comprising an elongated, resilient element of a length corresponding to the perimeter of said base in sealing engagement with the peripheral edge portions of said cover and said base.

8. An electrolytic cell of the mercury electrode type capable of being operated with a substantial fluid pressure difierential between its electrolyte inlet and outlet, which comprises a base carrying r a flowing mercury cathode, a cover on the base extending across the cell beyond the edges of said cathode, a solid electrode with a plane under-surface of such Width as to extend nearly across the cell, said electrode being rigidly supported in the cell by said cover so that vertical movements of the cover cause equal vertical movements in the electrode with its under-surface disposed a short distance above the upper surface of the liquid electrode and substantially parallel thereto, means combined with the cover for adjusting the height of the cover relative to the base, comprising a plurality of horizontal extensions on said cover which bear upon vertically adjustable support members carried by said base, and means for sealing the cover to the base for allowing sufficient relative vertical movement between the cover and the base to substantially alter the width of the inter-electrode space by such relative movement, comprising an elongated, resilient element of a length corresponding to the perimeter of said base in sealing engagement with the peripheral edge portions of said cover and said base.

9. An electrolytic cell as claimed in claim 1, wherein said means combined with the cover for adjusting the height of the cover relative to the base comprises a plurality of threaded bolts operatively engaging the cover and engaging threaded holes in said base.

CHARLES CARTER. JOHN V. S. GLASS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 271,906 Nolf Feb. 6, 1883 468,880 Le Sueur Feb. 16, 1892 748,985 Rink Jan. 5, 1904 1,346,849 Shaw July 20, 1920 2,104,678 Sorensen Jan. 4, 1938 2,328,665 Munson Sept. 7, 1943 FOREIGN PATENTS Number Country Date 173,520 Germany July 26, 1906 

1. AN ELECTROLYTIC CELL OF THE LIQUID ELECTRODE TYPE HAVING A BASE CARRYING A FLOWING LIQUID ELECTRODE, A COVER ON THE BASE EXTENDING ACROSS THE CELL BEYOND THE EDGES OF THE LIQUID ELECTRODE, A SOLID ELECTRODE RIGIDLY SUPPORTED IN THE CELL BY SAID COVER, SAID SOLID ELECTRODE HAVING A PLANE UNDER-SURFACE DISPOSED A SHORT DISTANCE ABOVE THE UPPER SURFACE OF THE LIQUID ELECTRODE AND SUBSTANTIALLY PARALLEL THERETO, SAID PLANE SURFACE EXTENDING CONTINUOUSLY ACROSS THE MAJOR PORTION OF THE CELL, MEANS COMBINED WITH THE COVER FOR ADJUSTING THE HEIGHT OF THE COVER RELATIVE TO THE BASE COMPRISING A PLURALITY OF HORIZONTAL EXTENSIONS ON SAID COVER WHICH BEAR UPON VERTICALLY ADJUSTABLE SUPPORT MEMBERS CARRIED BY SAID BASE, AND MEANS FOR SEALING THE COVER TO THE BASE FOR ALLOWING APPRECIABLE RELATIVE VERTICAL MOVEMENT BETWEEN THE COVER AND THE BASE COMPRISING AN ELONGATED, RESILIENT ELEMENT OF A LENGTH CORRESPONDING TO THE PERIMETER OF SAID BASE IN SEALING ENGAGEMENT WITH THE PERIPHERAL EDGES OF SAID COVER AND SAID BASE. 